In plain words
Thus I have declared to you jñāna more secret than all secrets. Reflect on it completely; then act as you choose.
Word for word
Thus knowledge more secret than the secret has been declared to you by Me; having reflected on this fully, as you wish, so do.
The commentators
Bhagavadgītābhāṣyam· bhāṣya
इति एतत् ते तुभ्यं ज्ञानम् आख्यातं कथितं गुह्यात् गोप्यात् गुह्यतरम् अतिशयेन गुह्यं रहस्यम् इत्यर्थः, मया सर्वज्ञेन ईश्वरेण । विमृश्य विमर्शनम् आलोचनं कृत्वा एतत् यथोक्तं शास्त्रम् अशेषेण समस्तं यथोक्तं च अर्थजातं यथा इच्छसि तथा कुरु ॥ ६३ ॥
In plain words
So this is the knowledge I've told you — deeper than any ordinary secret, a mystery beyond mysteries — spoken by Me, the one who knows all, the Lord Himself. Now sit with it. Turn it over completely, this teaching just as it was given, and everything it means. And then — do as you choose.
Close to the Sanskrit
Thus this knowledge has been declared, related, to you — more secret than what is [merely] secret, than what is [merely] to be concealed, that is, exceedingly secret, a mystery — this is the meaning — by me, the all-knowing one, the Lord. Having reflected — having done reflection, consideration — upon this teaching as it has been stated, completely, entirely, and upon the whole array of meanings as stated, as you wish, so do.